This invention relates generally to drive mechanisms that convert uni-directional rotational motion to reciprocal translation. An example of such a mechanism is a desmodromic drive, one use of which is as an actuator of an intake and/or an exhaust valve in an internal combustion engine.
A search conducted in connection with this invention developed U.S. Pat. Nos. 1,111,169; 1,490,656; 2,751,789; 3,492,880; 4,337,699; 5,099,805; 5,327,856; and 5,598,814; and U.K. Provisional Specification 22,822. Several of those patents are concerned with operation of engine intake and/or exhaust valves. Unlike a spring-biased engine valve that is forced open by a camshaft lobe and that relies on the spring force to close the valve, a desmodromic valve actuator assures both positive valve opening and positive valve closing.
At high engine speeds, a desmodromic actuator can be effective to prevent valve float that could otherwise when only spring force is used to close the valve. Hence, it is known to employ desmodromic valve actuators to operate valves of motorcycle engines whose top speeds can be much in excess of those typical of passenger car engines.
The present invention relates to novel desmodromic mechanisms, especially mechanisms for operating a valve such as an engine intake or exhaust valve. Various embodiments of novel mechanisms possessing various novel features are disclosed. The mechanisms are intended to provide both the performance, the efficiency, and the durability that are needed for use in the harsh environment of an automotive vehicle engine within the confines of limited space.
One generic aspect of the invention relates to a desmodromic drive for imparting reciprocal translation to a translatable member. The drive comprises a ring that rotates about an axis, a follower that reciprocates with straight line motion along the axis as the ring rotates for imparting reciprocal translation to the member, and a coupling of the ring to the follower that causes the follower to reciprocate along the axis as the ring rotates. The coupling comprises an endless cam track on one of the ring and the follower and an element on the other of the ring and the follower that rides along the cam track as the ring rotates. A follower guide guides the follower for axial motion along the axis. The drive includes a roller that defines a circumferential relation of the follower to the follower guide about the axis. The roller may be either a circular cylinder or a sphere.
Another aspect relates to a desmodromic drive for imparting reciprocal translation to a valve element of a valve. The drive has a ring, a follower, a coupling, and a follower guide, as described above, and a feature that defines a circumferential relation of the follower to the follower guide about the axis. The valve has a stem through which the reciprocation of the follower imparts reciprocal translation to the valve element. A spiral wave spring biases the stem relative to the follower to allow for lost-motion over-travel of the follower relative to the stem.
Still another aspect relates to a desmodromic drive mechanism that imparts reciprocal translation to a valve element of a valve. The mechanism comprises a ring, a follower, a coupling, a follower guide, a feature that defines a circumferential relation of the follower to the follower guide about the axis, and a stem, as described above. The valve element closes against the valve seat concurrent with the element that rides along the cam track riding along a certain segment of the cam track. The follower comprises a central hub concentric with the axis and an arm that extends radially outward from the hub to the element that rides along the cam track, and the arm is arranged to flex as the element that rides along the cam track rides along the certain segment of the cam track and thereby cause the valve element to be forced against the valve seat when the valve element closes against the valve seat.
Still another aspect relates to a desmodromic drive for imparting reciprocal translation to a translatable member and comprising a ring, a follower, a coupling, and a follower guide. The follower comprises a central hub concentric with the axis and two pairs of arms that extend radially outward from the hub. A first pair of the arms extend in opposite directions to respective elements that ride along an endless cam track on the ring. The cam track comprises two identical segments each running along a respective semi-circumference of the ring. The follower guide comprises two axially extending grooves that are disposed facing and diametrically opposite each other about the axis and circumferentially between the two arms of the first pair. Each arm of a second pair of the arms extends from the hub toward a respective one of the two grooves of the follower guide and carries a respective sphere that is spring-biased radially outwardly of the axis to ride in the respective groove as the follower axially reciprocates.
Further aspects will be seen in various features of presently preferred embodiments of the invention that will be described in detail.